[7439-95-4]  · Mg  · Magnesium-Methanol  · (MW 24.31) (MeOH)

[67-56-1]  · CH4O  · Magnesium-Methanol  · (MW 32.05)

(reducing system for a,b-unsaturated nitriles, amides, esters, ketones, imines, halides, sulfones, and azides)

Physical Data: Mg: mp 651 °C; bp 1107 °C; d 1.74 g cm-3. MeOH: bp 64.65 °C; d 0.796 g cm-3.

Form Supplied in: Mg: powder, ribbon, chips, shavings; widely available. MeOH: anhydrous and reagent grade widely available.

General Reductions.

Reductions with Magnesium in methanol are usually accomplished simply by stirring the reactants at ambient temperature using an excess (5-40-fold) of magnesium. There is often an induction period, probably needed to cleanse the surface of the magnesium for efficient reaction. The ensuing reaction with methanol is exothermic, and may or may not require cooling in an ice bath.1-8

Carbon-Carbon p-Bond Reductions.

The dissolving metal reducing system generated by this reagent combination provides convenient electron transfer/protic source reductions of a,b-unsaturated nitriles1 (eq 1),1d amides2 (eq 2),2c and esters3 (eq 3)3b to the saturated derivatives. Conjugated alkynic esters (but not acids) also afford saturated products (eq 4),4 while a,b-unsaturated ketones and imines give the corresponding saturated alcohols or amines, respectively, resulting from concomitant reduction of the carbonyl or imine group.5 However, a,b-unsaturated ketones further conjugated to furan rings are selectively converted to the saturated ketones (eq 5)6 without affecting the carbonyl.6 Deuterium may be introduced by using MeOD as the solvent (eq 4).3b,4 Isolated double and triple bonds are not affected4,1d,2,3c unless conjugated to two (but not one) phenyl groups (eq 6).4,7

Functional Group Reductions.

The reagent system effectively and selectively reduces alkyl and aryl iodides and bromides to hydrocarbons.5a,8a Vinyl bromides are reduced if conjugated to an aromatic ring, while only benzylic and allylic chlorides are reduced (eq 7).8a Deuterium may be incorporated using MeOD as the solvent (eq 8).8a Aryl chlorides convert to hydrocarbons using a similar system involving magnesium powder and isopropanol in refluxing decahydronaphthalene.8b

Mono- and bisulfones are effectively desulfonated with magnesium-methanol to afford mostly alkenes (with 1,2-disulfones, eq 9)9 or hydrocarbons (with 1,1-disulfones or monosulfones, eq 10).9 In these cases, the magnesium was preactivated by treatment with dilute HCl.

Alicyclic, benzylic, and allylic azides are efficiently reduced to the corresponding amines with magnesium powder in methanol (eq 11).10 Aromatic nitro compounds are reduced to mixtures of azo and azoxy derivatives.5a Reduction of an N-magnesium imine to the amine has been reported.11

This reagent combination provides a convenient, mild, chemoselective, and inexpensive dissolving metal process for reduction of double bonds in conjugated esters, nitriles, and amides and for selective reductions of alkyl and aryl iodides and bromides.

1. (a) Profitt, J. A.; Watt, D. S.; Corey, E. J. JOC 1975, 40, 127. (b) Freerksen, R. W.; Pabst, W. E.; Raggio, M. L.; Sherman, S. A.; Wroble, R. R.; Watt, D. S. JACS 1977, 99, 1536. (c) Camps, P.; Ortuno, R. M.; Serratosa, F. T 1976, 32, 2583. (d) Heissler, D.; Riehl, J. J. TL 1979, 3957. (e) Dinizo, S. E.; Freerksen, R. W.; Pabst, W. E.; Watt, D. S. JOC 1976, 41, 2846. (f) Takano, S.; Yamada, S.; Numata, H.; Ogasawara, K. CC 1983, 760. (g) Osborn, M. E.; Pegues, J. F.; Paquette, L. A. JOC 1980, 45, 167. (h) Geze. M.; Blanchard, P.; Fourrey, J. L.; Robert-Gero, M. JACS 1983, 105, 7638. (i) Freerksen, R. W.; Selikson, S. J.; Wroble, R. R.; Kyler, K. S.; Watt. D. S. JOC 1983, 48, 4087. (j) Maity, S. K.; Mukherjee, D. T 1984, 40, 757. (k) Kandil, A. A.; Slessor, K. N.; JOC 1985, 50, 5649. (l) Braisch, T. F.; Fuchs, P. L. SC 1985, 15, 549. (m) Garratt, P. J.; Doecke, C. W.; Weber, J. C.; Paquette, L. A. JOC 1986, 51, 449. (n) Paquette, L. A.; Okazaki, M. E.; Caille, J.-C. JOC 1988, 53, 477.
2. (a) Brettle, R.; Shibib, S. M. TL 1980, 21, 2915. (b) Brettle, R.; Shibib, S. M. JCS(P1) 1981, 2912. (c) Palmisano, G.; Danieli, B.; Lesma, G.; Riva, R.; Riva, S. JOC 1984, 49, 4138. (d) Ainscow, R. B.; Brettle, R.; Shibib, S. M. JCS(P1) 1985, 1781.
3. (a) Youn, I. K.; Yon, G. H.; Pak, C. S. TL 1986, 27, 2409. (b) Hudlicky, T. Zingde, G. S.; Natchus, M. G. TL 1987, 28, 5287. (c) Hudlicky, T.; Natchus, M. G. Zingde, G. S. JOC 1987, 52, 4641. (d) de Laszlo, S. E.; Ley, S. V.; Porter, R. A. CC 1986, 344 (with ultrasound).
4. Hutchins, R. O.; Suchismita; Zipkin, R. E.; Taffer, I. M.; Sivakumar, R.; Monaghan, A.; Elisseou, E. M. TL 1989, 30, 55.
5. (a) Zechmeister, L.; Rom, P. LA 1929, 468, 117. (b) Zechmeister, L.; Truka, J. CB 1930, 63, 2883.
6. Dominguez, C.; Csaky, A. G.; Plumet, J. TL 1991, 32, 4183.
7. Profitt, J. A.; Ong, H. H. JOC 1979, 44, 3972.
8. (a) Hutchins, R. O.; Suchismita; Zipkin, R. E.; Taffer, I. M. SC 1989, 19, 1519. (b) Bryce-Smith, D.; Wakefield, B. J. OSC 1973, 5, 998.
9. (a) Brown, A. C.; Carpino, L. A. JOC 1985, 50, 1749. (b) Kündig, E. P.; Cunningham, Jr., A. F. T 1988, 44, 6855.
10. Maiti, S. N.; Spevak, P.; Reddy, A. V. N. SC 1988, 18, 1201.
11. Krepski, L. R.; Jensen, K. M.; Heilmann, S. M.; Rasmussen, J. K. S 1986, 301.

Robert O. Hutchins

Drexel University, Philadelphia, PA, USA

MaryGail K. Hutchins

King of Prussia, PA, USA

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